This invention relates, in general, to semiconductor structures, and more particularly, to a method of manufacturing isolation structures in semiconductor integrated circuits.
Semiconductor devices on integrated circuits must be electrically isolated from each other. The most common isolation technique is by localized oxidation of silicon (LOCOS). A disadvantage of this technique is that the surface area in which the semiconductor devices are to be fabricated, or active area, is reduced by the lateral encroachment of the isolation oxide formed. The problem of lateral encroachment is well known in the art.
In the LOCOS process a nitride mask is used to prevent oxidation of the active area. A pad oxide layer is formed under the nitride mask to relieve stress. The amount of encroachment is thought to be related to the pad oxide thickness near the edge of the nitride mask. The oxidation under the nitride mask occurs when oxygen diffuses through the pad oxide layer and reacts with the silicon underlying it.
A method of minimizing the lateral encroachment entails reducing the thickness of the pad oxide layer in order to reduce the cross-sectional area available for oxygen diffusion. This vertical scaling is limited by the stress of nitride. As a general rule, the ratio of oxide to nitride thicknesses is maintained at greater than or equal to 1/3. If the nitride is thinned, for the same pad oxide thickness, lateral encroachment also increases. One approach to reduce the pad oxide thickness without reducing the nitride thickness requires a polysilicon layer to be formed between the nitride layer and the pad oxide in order to relieve the compressive stress in the substrate brought about by reducing the pad oxide thickness. Although a slight reduction of oxide encroachment is achieved, the process is difficult to control and requires an additional step to remove both the nitride and the polysilicon following the oxidation process.
A LOCOS method has been proposed by Roth et al in Pat. No. 4,927,780, entitled "Encapsulation Method for Localized Oxidation of Silicon," which was issued on May 22, 1990, which is hereby incorporated herein by reference. Further improvements will be realized by the present invention, as will be evident from the description which follows.
Other techniques to reduce lateral encroachment have been tried, but fail to provide a method which is cost effective, manufacturable, and results in the production of a defect-free semiconductor structure.
Accordingly, it is an object of the present invention to provide an improved LOCOS process.
Another object of the present invention is to provide an improved LOCOS process having minimized lateral encroachment.
Yet another object of the present invention is to provide an improved LOCOS process with a reduced defect density.